During the last year we have shown the following: 1) We have used the R-loop technique in analyzing hybrids formed between linear polyoma DNA and polyoma late poly(A)-containing cytoplasmic RNA. Three species of late viral RNAs were identified and mapped on the physical map of polyoma DNA. Another conclusion of these studies is that the three late polyoma RNAs are spliced. 2) We have provided evidence that at least one of the Moloney murine leukemia virus RNAs is spliced. Furthermore, based on the observation that spliced viral RNA formed circle pon annealing with cDAN, we have suggested a new method to quantitate and determine the lengths of spliced viral RNAs in retrovirus infected cells. 3) Electronmicroscopic analysis of hybrid structures formed between nuclear and cytoplasmic minute virus of mice (MVM) RNAs and genomic single stranded MVM DNA provided evidence that the most abundent MVM RNA is spliced and that splicing occurs on the precursor poly(A) plus nuclear RNA. 4) We have shown the existence of a mechanism of attenuation during SV40 late transcription. 5) We have developed a method by which RNA polymerase II can be rapidly obtained from cell nuclei. 6) We have localized the internal 6mAs found in SV40 nuclear lat RNAs on the E and L - SV40 DNA strands.